Biomaterial enables proton-based FET

Researchers from the University of Washington have designed a transistor that can control proton current.

Devices that connect with the human body's processes are being explored for biological sensing or for prosthetics, but they typically do so with electrons. However, materials scientists at the University of Washington (UW) have developed a transistor that, instead of electrons, uses protons that enable devices to communicate directly with living things.

"So there's always this issue, a challenge, at the interface—how does an electronic signal translate into an ionic signal, or vice versa?" said Marco Rolandi, UW assistant professor of materials science and engineering and lead author of the study. "We found a biomaterial that is very good at conducting protons and allows the potential to interface with living systems."

In the body, protons activate 'on' and 'off' switches and are key players in biological energy transfer. Ions open and close channels in the cell membrane to pump things in and out of the cell. Animals including humans use ions to flex their muscles and transmit brain signals. A machine that was compatible with a living system in this way could, in the short term, monitor such processes. Someday it could generate proton currents to control certain functions directly.

A first step towards this type of control is a transistor that can send pulses of proton current. The prototype device is a field-effect transistor (FET). The UW prototype is the first such device to use protons. It measures about 5µm wide, roughly a twentieth the width of a human hair.

"In our device, large bioinspired molecules can move protons, and a proton current can be switched on and off, in a way that's completely analogous to an electronic current in any other FET," Rolandi said.

The device uses a modified form of the compound chitosan originally extracted from squid pen, a structure that survives from when squids had shells. The material is compatible with living things, easily manufactured and can be recycled from crab shells and squid pen discarded by the food industry.

First author Chao Zhong, a UW postdoctoral researcher, and second author Yingxin Deng, a UW graduate student, discovered that this form of chitosan works remarkably well at moving protons. The chitosan absorbs water and forms many hydrogen bonds, enabling protons to hop from one hydrogen bond to the next.

Computer models of charge transport developed by co-authors M.P. Anantram, a UW professor of electrical engineering, and Anita Fadavi Roudsari at Canada's University of Waterloo, were a good match for the experimental results.

"So we now have a protonic parallel to electronic circuitry that we actually start to understand rather well," Rolandi continued.

Applications in the next decade or so would likely be for direct sensing of cells in a laboratory, added Rolandi. The current prototype has a silicon base and could not be used in a human body. Longer term, however, a biocompatible version could be implanted directly in living things to monitor, or even control, certain biological processes directly.

The other co-author is UW materials science and engineering graduate student, Adnan Kapetanovic.

On the left is a coloured photo of the UW device overlaid on a graphic of the other components. On the right is a magnified image of the chitosan fibres. The white scale bar is 200nm.

What is a transistor?
An active semiconductor device with three or more terminals. A device used to amplify voltage, current or function as an ON/OFF switch. Transistor is the fundamental building block of computer circuitry.